3GPP SA6 is currently specifying Mission Critical Push To Talk (MCPTT) over LTE application. MCPTT replicates the push to talk service behaviour provided in legacy systems like TETRA or P.25. This service is typically used for Public Safety applications.
The service has typically the following characteristics:                Two or more users participate in a group communication;        Users have to request for permission to talk, traditionally by pressing a button on their handset;        Only one user can talk at a certain time, all others are just listeners.        
One of the architectural alternatives for so called “on network operation” describes that the MCPTT application runs on top of the IP multimedia core network (IM CN) subsystem. The MCPTT architecture and procedures will be described in 3GPP TS 23.179 and the IMS architecture and procedures are described in 3GPP TS 23.228. Generally speaking, MCPTT allows a user to talk to other users of a group similar to a Walkie Talkie communication service.
MCPTT supports two types of calls, namely a group call and a private call. The group call allows one speaker to talk to a group of users, while the private call allows two MCPTT users to talk to each other.
Typically, the MCPTT server can leverage MCPTT specific user identities and alias information to setup MCPTT calls, both for group calls as for private calls. MCPTT private calls will be routed via the MCPTT application server.
The MCPTT public user identity is an identity that is used to setup calls to a certain user. The identities are of the form of a URI and may look like                SIP URI fireman-joe-smith@fire.chicago.us        